And I'm not sure what sort of legal safeguards there are as an investor / mortgagor when the borrower still holds title and could potentially run up other debt against the asset.

How to identify your biases and safeguard

Making sure you have the appropriate documentation for the appropriate kind of loan with the appropriate lending terms is the easiest way to safeguard you money (besides not lending too much money on a project). 

Is there any strategy to do such a transaction, proceeds of rental sale, to buy undeveloped land with intent on building an Airbnb small cabin, that would safeguard our net? 

I keep my LTV at 50-65% of the purchase price or BPO value, and even then I feel safe, but I also have several other safeguards in place, or I wouldn't lend my money.

Point out the safeguards for them and aspects of servicing the loan by a professional. 

Account Closed A point I hadn't read being made is that it is not an A-or-B option of agent or no agent, there are more forms of agency a person can utilize such as a facilitator agreement so that in the transaction it's a safeguard all legal contracts are being correctly utilized to effect a legal, known transaction vs some sneaky terms snuck in or more commonly, important legal measures left out that come back to bite a person in the end.

Like all things investing (and perhaps in life) diversification is prudent and safeguards from pitfalls, so in my personal investing when the portfolio gets a little too heavy in old properties with high CapEx liabilities I look to sprinkle in some new properties with low CapEx liabilities.   

In the end you just have to ask yourself if you have the time to manage it or manage your peacemealed portions and then you also have to factor how effectively you'll be able to manage it and if you'll be able to 'safeguard' the property properly from a distance.

While I do have good relationships with all my sellers, I safeguard myself by depositing my payments directly in their bank accounts.